The Capris have a huge cut. We were able to put one on my 50lb 4 yo on the largest setting one night when his super undies werent ready for bedtime. I have a light pink one with some of the edge stitching coming undone (see photo) that I could send you for $5 if you want to just try the fit? They are usually $15 on the FSOT so could resell it for $8-$9 (since it's got the loose stitches) and make all your money back (including fees and shipping) if the fit isn't right. Or I could FFS it to you, and you could send it back to me too.
You could also try maybe the 3T Gerber plastic pants over prefolds? Is 3T a good size or do you need 4T?